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<title>Serialization - Derivation from an Existing Archive</title>
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      <h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3>
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      <h1 align="center">Serialization</h1>
      <h2 align="center">Derivation from an Existing Archive</h2>
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<h3>Log Archive</h3>
It may happen that one wants to create a new archive class by derivation from one
of the included ones. Included is a sample program that shows how to derive a 
new archive from one of the ones included with the library.  The first example is
<a href="../example/demo_log.cpp" target="demo_log_cpp">
demo_log.cpp</a>.
<p>
This derivation from the xml archive writes output in xml without the extra
information required to read the data back into the application.  It might be
used to export one's data as simple xml for other applications or for logging
data while debugging.
<p>
To this end it is derived from the included xml archive and the save functions for
some types are specialized for this application.  
<p>
The serialization library is
implemented using the <b>C</b>uriously <b>R</b>ecurring <b>T</b>emplate 
<b>P</b>attern (<b>CRTP</b>).  Also, all common code is factored out into
separate modules to minimize code repetition. This makes derivation from
an existing archive less straightforward than it would otherwise be.
<p> 
This example illustrates several issues that have to be addressed when doing
something like this
<ol>
    <li><i>It is derived from</i> <code style="white-space: normal">xml_oarchive_impl<log_archive></code> 
        <b>NOT</b> <code style="white-space: normal">xml_oarchive</code> <br>
As described in the comments in
<a href="../../../boost/archive/xml_oarchive.hpp" target="xml_oarchive_hpp">xml_oarchive.hpp</a>.
<code style="white-space: normal">xml_oarchive</code> really a shorthand name for 
<code style="white-space: normal">xml_oarchive_impl&lt;xml_oarchive&gt;</code>.  So we should derive
from <code style="white-space: normal">xml_oarchive_impl&lt;log_archive&gt;</code> rather
than <code style="white-space: normal">xml_oarchive</code>.
<pre><code>
class log_archive :
    // don't derive from xml_oarchive !!!
    public xml_oarchive_impl&lt;log_archive&gt;
{
...
</code></pre>
    <li><i>Note the</i> <code style="white-space: normal">log_archive</code> <i>between the</i> &lt;&gt;
This is required so that base classes can downcast their <code style="white-space: normal">this</code> pointer
to the most derived class.  This is referred to as <b>C</b>uriously <b>R</b>ecurring
<b>T</b>emplate <b>P</b>attern (<b>CRTP</b>) <a href="bibliography.html#11">[11]</a>.  
It is used to implement static polymorphism.
    <li><i>Base classes need to be explicitly given access to the derived class.</i>
This can be done by making members public or by including friend declarations for
the base classes.
<pre><code>
    friend class detail::common_oarchive&lt;log_archive&gt;;
    friend class basic_xml_oarchive&lt;log_archive&gt;;
    friend class boost::serialization::save_access;
</code></pre>

    <li><i></i>Reread <a target="detail" href="headers.html#archiveinternals">Archive Internals</a>.
This describes the class hierarchy so that you know what to override.
    <li><i>Base class functions will usually need to be explicitly invoked.</i>
We commonly specialize the function name <code style="white-space: normal">save_override</code> 
for saving primitives. Usage of a function name in a derived class
"hides" similarly named functions of the base class.  That is, 
function name overloading doesn't automatically
include base classes.  To address this, we can use:
<pre><code>
    using xml_oarchive_impl&lt;derived_t&gt;::save;
    void save(const unsigned int t);
    ...
</code></pre>
which should work on conforming compilers. However, I have found
that the following equivalent works on more compilers.
<pre><code>
    // default fall through for any types not specified here
    template&lt;class T&gt;
    void save(const T & t){
        xml_oarchive_impl&lt;derived_t&gt;::save(t);
    }
    void save(const unsigned int t);
    ...
</code></pre>
so it's what I use.
    <li><i>Template definitions of base classes may have to be explicitly instantiated.</i>
    The demo includes
<pre><code>
// explicitly instantiate for this type of binary stream
#include &lt;boost/archive/basic_binary_oprimitive.ipp&gt;
</code></pre>
for just this purpose.  Failure to include required template definitions
will result in undefined symbol errors when the program is linked.
    <li><i>Without alteration, this class cannot be further derived from.</i><br>
Base classes using <b>CRTP</b> must be templates with a parameter corresponding to
the most derived class.  As presented here, this class doesn't qualify, so
it cannot be used as a base class.  In order to derive further from this class,
it would have to be reorganized along the lines of the original <code style="white-space: normal">xml_oarchive</code>.
Specifically, it would look something like:
<pre><code>
template&lt;class Archive&gt;
class log_archive_impl :
    // don't derive from xml_oarchive !!!
    public xml_oarchive_impl&lt;Archive&gt;
{
    ...
);

// do not derive from this class !!!
class log_archive : 
    public log_archive_impl&lt;log_archive&gt;
{
public:
    log_archive(std::ostream & os, unsigned int flags = 0) :
        log_archive_impl&lt;xml_oarchive&gt;(os, flags)
    {}
};
</code></pre>

</ol>  

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<p><i>&copy; Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004. 
Distributed under the Boost Software License, Version 1.0. (See
accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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